Hi, this is a bit of a strange situation so I'm really hoping someone can help me out. I shoot a lot of concerts, theatre, etc. where flash is not required but generally haven't done much indoor portrait and private event photography. So I've really only started using flash this past year and am still getting the hang of it. At the moment, my main setup is a 5D III with a Speedlite 380ex using the hot shoe with a basic plastic diffuser. I had a short shoot in a dimly lit room today (just a bride and groom signing papers and exchanging rings, nothing fancy) and I used a 24-70mm 2.8 II lens, using a colleague's recommended settings for indoor event flash photography, bouncing the flash off the ceiling. At first, it looked just fine. The lighting was even and nicely colored and I was really happy with it. Then, after about 5 minutes, without changing the settings, location or anything else, the photos very suddenly started coming out incredibly dark and poorly colored. I realize that sometimes the flash doesn't fire properly and/or needs a moment to reload but in this case it WAS firing. Yet the pictures were almost pitch black. I had to bump up the ISO to 4000 just to get the lighting to acceptable levels, but the coloring was extremely red. I took a bunch of pictures and the results didn't change. The flash kept firing but the images had completely lost the earlier balanced look. I've attached 3 sample photos (screenshots of the RAW files straight out of the camera; faces are pixellated but otherwise unedited). As you can see, the first image looks normal, the second is with identical settings and the flash did fire but it's completely dark, and the third is what it looked like after I bumped up the ISO to compensate. If it helps, the RAW Properties are as follows: EX 1 (https://www.dropbox.com/s/6cil3bsu7r166rp/ex1.jpg?dl=0) ISO 400 F-stop f/2.8 Exposure time 1/160 sec Exposure bias 0 step Exposure program Manual Metering Mode Pattern Flash mode Flash; compulsory Focal length 24mm EX 2 (https://www.dropbox.com/s/7192qar5p9fz2i7/ex2.jpg?dl=0) Same as above, but ISO 500 EX 3 (https://www.dropbox.com/s/ca2qipf2f5kjtml/ex3.jpg?dl=0) Same as Image 1 but ISO 4000 and Focal length 70mm. I have no idea what on earth happened here. I can see this sort of thing happening if I changed the settings or if the flash stopped working or if we switched to a different location but there was literally no change - from one shot to the next the photos just suddenly started coming out very dark and discolored. It wasn't the batteries because brand new ones had just been put into the Speedlite when the shoot began and as I said, the flash was indeed going off. Granted, I've only done about 10 shoots that used flash but this has never happened to me before and was completely bizarre. Can anyone tell me what might have caused this and what I should look out for or do in the future if it happens again?